Process and apparatus to control the integrity of a planar substrate

ABSTRACT

The process comprises the following steps: detection of the passage of an edge of the substrate by a first trigger, detection of the passage of said edge of the substrate at least at a first selected checkpoint on the substrate; control of the presence of the detection of the edge of the substrate at said at least first checkpoint between said detection by said triggers and generation of an integrity check failed message in the absence of the detection of the edge of the substrate at said checkpoint.

The present invention concerns a process to control the integrity of aplanar substrate, for example sheets of paper for securities.

The present invention also concerns a device suitable for carrying outthe process according to the invention.

In the field of printing machines, in particular for securities such abanknotes, checks and other similar printed matter, many qualitycontrols are carried out during the entire printing process. Indeed, itis very important to ensure a high quality of production, especially inthe field of securities, and hence the precise controls. Such controlscan take the form of a check of the quality of the printing, of therecto-verso register of prints on the sheet of substrate etc. It is ofcourse also necessary to control the shape of the substrate, i.e. tocheck that the substrate, for example a paper substrate, is not torn orfolded.

In known methods and devices of the prior art, the quality control of apaper substrate during the printing process is made by a tactile processby contacting the entire surface of the sheet of paper. Such a methodhas several drawbacks, i.e. the fact that it contacts the substrate andalso the fact that is has to be able to follow the speed of the sheetsbeing controlled. Moreover, a direct contact with the substrate may markor damage said substrate, causing defects in the printing process.

Other devices use photocells to control the sheet edge, said cells beingtriggered by the machine. They are thus speed and paper size dependent.

It is therefore an aim of the invention to improve the known methods ofcontrol and machines able to carry out said methods.

More specifically, an aim of the invention is to provide a simple methodto control the integrity of planar substrates, such as sheets of paperwhich is independent from speed and paper size.

Another aim of the invention is to provide a simple and effectivemachine to check the integrity of planar such as sheets of paper.

The invention is characterised by the features defined in the claims.

Further characterizing features and advantages of the present inventionwill become apparent from the following detailed description, given byway of non-limitative examples, and illustrated by the accompanyingdrawings, in which

FIG. 1 shows a bloc-diagram of the process according to the invention.

FIG. 2 shows schematically a control of a substrate, such as a sheet.

FIG. 3 shows a diagram of detection for the leading edge of a substrate.

FIG. 4 shows a diagram of detection for the trailing edge of asubstrate.

FIG. 5 shows a circuit suitable for carrying out the detection.

The bloc-diagram of FIG. 1 discloses the steps of the process accordingto the invention and applied to a planar substrate being controlled.

As the first step, a first detection is carried out, namely a detectionof the passage of an edge of the substrate by a first trigger. Thisfirst step can be used to activate the process of detection by a triggersignal. Then, the second step includes the detection of the passage ofthe edge of the substrate at least at a first selected checkpoint on thesubstrate issuing a first checkpoint signal. Preferably, since it is thecorners of the sheet of substrate that are usually torn or folded, theat least one checkpoint is preferably situated close to a corner of thesubstrate. Most preferably, because a planar substrate carries fourcorners (two at the leading edge and two at the trailing edge), one usestwo checkpoints placed in the area of the corners of the substrate, oneach side of the transporting direction of the substrate.

Then, a third detection is carried out by a second trigger of thepassage of the edge of the substrate that has been detected in the firststep, the detection of this trigger signal terminating in principle thedetection steps of the process.

After that, there is a control of the presence of the detection of theedge of the substrate at said at least first checkpoint between saiddetection by said triggers by using the above-mentioned issued signals.Indeed, an idea of the process is to detect the passage of the edge ofthe substrate at one or more selected checkpoints between the detectionof triggers or at least after the detection of a first trigger.Preferably, as indicated above, the checkpoint or checkpoints areselected to be placed where the substrate has the most probably a defect(in corners) and the trigger (or triggers) is (are) placed where thesubstrate has the least probably a defect.

If the detection signal of the checkpoint or checkpoints has not givenany result, i.e. if a checkpoint has not emitted a change signal after afirst trigger signal or between the detection of both trigger signals ora checkpoint signal is detected after or before said first triggersignal, then there is the generation of an integrity check failedmessage (“error message” in FIG. 1). If the checkpoint or checkpointssignals have been properly detected, that is in the proper order afterthe first trigger signal, and before the second trigger if any, thenthere is an OK message.

The detection process according to the invention is finter describedwith reference to FIG. 2 in which a substrate 1, for example a sheet, inmovement in the direction of the arrow, passes under a detection deviceaccording to the invention. The detection device comprises a firsttrigger 2, a second trigger 5 and checkpoints 3, 4. For the sake ofsimplicity, a detection device is represented schematically on theleading edge 6 of the sheet 1 and also on the trailing edge 7 of thesheet. It is clear however that only one detection device is sufficientto carry out the process of the invention for both edges when thesubstrate is being trasported in the direction of the arrow andexplanations will be given for the leading edge 6 first. Further, it isalso possible to use more than two checkpoints, for example a row of aplurality of checkpoints able to control the entire width of the sheet,the embodiment of FIG. 2 being only used as a non-limiting example.

If, for example, the left corner 8 of the substrate is folded or ismissing, the first trigger 2 will detect the leading edge 6 and output afirst trigger signal, checkpoint 3 will detect the leading edge 6 andoutput a corresponding checkpoint signal, and then second trigger 5 willdetect the leading edge and output another trigger signal. A secondcheckpoint detection signal will be missing, or arrive a certain timeafter the detection of the second trigger 5 signal, indicating thepassage of the leading edge 6 in the area of the second checkpoint, thusgenerating an error message, because the detection of the checkpointsignal was not made between the two triggers 2 and 5 signals.

The same integrity check may be applied to the trailing edge 7 of thesubstrate 1, as schematically represented in FIG. 2. With the substrate1 being transported, in the direction of the arrow, the trailing edge 7of the substrate reaches the trigger 2 first. This trigger 2 hencedetects a change in the background, for example from a clear backgroundto a dark background, and initiates the control process with a triggersignal.

Because of its displacement, the trailing edge 7 of the substrate 1reaches then checkpoint 3 which detects the change in the background,for example from a clear background to a dark background with opticalmeans and outputs a first checkpoint signal. The substrate 1 still beingdisplaced, the trailing edge 7 passes checkpoint 4 where a change in thebackground is also detected (for example from a clear background to adark background with optical means) and a second checkpoint signal isoutput. Finally, trailing edge 7 passes second trigger 5, againdetecting a change in the background whereby the detection is terminatedat least temporarily by a second trigger signal.

As for the leading edge 6, by knowing the speed of the substrate and therelative position of triggers 2, 5 and checkpoints 3, 4, in thedirection of motion of the substrate, it is in addition possible todetermine the timing sequence of detection of the checkpoint 3, 4signals relative to the trigger signals. Any absence of a checkpointsignal detection between the two triggers 2, 5 signals or detection of acheckpoint signal after the second trigger 5 signal, or even absence ofdetection of a checkpoint signal indicates a defect in the substrate,for example a damaged corner.

The distance, in the direction of movement of the substrate 1, betweenthe different detection points (triggers 2, 5 and checkpoints 3, 4) maybe varied or adjusted to increase the speed of detection.

The process according to the invention is explained in a more detailedmanner with reference to FIGS. 2 to 4, in which an optional secondtrigger 5 is used. The planar substrate 1 being transported in thedirection of the arrow, the leading edge 6 reaches trigger 2 first ofthe fixed detection device as schematically represented by the dashedline perpendicular to the directions of transportation in FIG. 2. Thus,trigger 2, for example realised by an optical detector, detects a changein the background, for example but not limited thereto, a passage fromblack (background without substrate) to white (substrate in thebackground), i.e “no paper” to “paper” which initiates the detectionprocess by outputting a trigger signal S1. This detected change isrepresented in FIG. 3 by the passage from “no paper” to “paper” of S1.With the substrate 1 moving, another detection is carried out atcheckpoints 3 and 4, which are placed close to opposite corners of thesubstrate, but are offset with respect to each other in the direction ofmovement of the substrate 1, in order to be able to distinctly detectthe leading edge 6 of the substrate 1 passing one checkpoint after theother, for example checkpoint 3 and then checkpoint 4, if checkpoint 3is closer to trigger 2 in the direction of motion of the substrate 1.The detection principle is similar to the one carried out for trigger 2,for example by detecting optically a change in the background from darkto clear (black to white), i.e. “no paper” to “paper”, the aim being tobe able to detect the passage of the edge of the substrate in thecorners covered by checkpoints 3 and 4 and two checkpoint signals S2, S3are output. This detection is represented in FIG. 3 by the changes in S2and S3, illustrating a passage from “no paper” to “paper”.

If the trigger 2 detects a change from “no paper” to “paper” but one ofthe checkpoints, or even both, has already output a signal change from“no paper” to “paper”, then the sheet area controlled must be considereddefective. This may be represented by the Boolean operation S1

{overscore (S2)}

{overscore (S3)}=

({overscore (S1)}

S2

S3)

Finally, with the substrate still in movement with respect to thedetection device, the leading edge 6 of the substrate 1 reaches secondtrigger 5 that reacts in the same manner as first trigger 2, bydetection of a change in the background (dark to clear for example) i.e.“no paper” to “paper”, outputs a second trigger signal S4 and thecontrol process can be ended. This detected change is represented inFIG. 3 by the change in S4, i.e. passage from “no paper” to “paper”.

If the trigger signal S4 changes from “no paper” to “paper” and one ofthe checkpoint signals S2, S3 has not changed from “no paper” to“paper”, then the sheet area has to be considered defective (for examplehole, missing edge etc): this may be represented by the Booleanoperation {overscore (S4)}

S2

S3=

(S4

{overscore (S2)}

{overscore (S3)}).

For the trailing edge of the substrate, reference is now madespecifically to FIGS. 2 and 4. Trigger 2 first detects the passage ofthe edge of the substrate and changes from “paper” to “no paper”, thischange being represented by trigger signal Si in FIG. 4. After thedetection by the first trigger, first checkpoint 3 and then secondcheckpoint 4 have to output a signal, respectively S2 and S3 to indicatethe passage of the trailing edge, i.e. a passage from “paper” to “nopaper”.

If the trigger signal S1 changes from “paper” to “no paper” and one ofthe checkpoints 3, 4 signals has already changed from “paper” to “nopaper” (signals S2 or S3 in FIG. 4), then the substrate area has to beconsidered defective (for example missing edge, crease) and this may berepresented by the Boolean operation {overscore (S1)}

S2

S3=

(S1

{overscore (S2)}

{overscore (S3)}).

The trigger 5 finally detects the end of the sheet edge, hence the endof monitoring. If this trigger 5 signal changes from “paper” to “nopaper” (signal S4 in FIG. 4) and one of the checkpoints 3, 4 signals hasnot changed from “paper” to “no paper” (signals S2 or S3 in FIG. 4),then the substrate area controlled by the triggers has to be considereddefective (for example missing edge, crease) and this may be representedby the Boolean operation S4

{overscore (S2)}

{overscore (S3)}=

({overscore (S4)}

S2

S3)

Therefore, the system provides four detected change signals with twodetections carried out in places potentially having a defect between twotriggers placed where the substrate has most probably no defect.

By knowing the speed of transport of the substrate, the positions andrelative distances between triggers and checkpoints in the direction oftransport of the substrate 1, it is easy to calculate the timing of thedetection signals for the triggers 2, 5 and the checkpoints 3, 4, i.e.once the first trigger has detected the leading edge 6 of the substrate,after how much time first checkpoint 3 and second checkpoint 4 have todetect said leading edge 6 if the substrate has no defect, and then whenthe second trigger has to detect said leading edge 6. Accordingly, thedetection of both checkpoint signals between the detection of thetrigger signals allows controlling the integrity of the substrate 1.

As indicated above, in the process according to the invention, it ispossible to carry out the integrity check either on the leading edge ofa substrate or on the trailing edge, or even on both edges with the samedetection device.

An example of a control device for carrying out the process according tothe invention is described with reference to FIGS. 2 and 5.

This device comprises at least three detectors, one trigger 2 and twocheckpoints 3, 4, preferably four detectors 2 to 5 as shown in FIG. 2,arranged to detect the leading edge 6 or trailing edge 7 of a substrate1 at selected points, as explained above with reference to the processof the invention. For example, a detector is used as the first trigger2, a detector as first checkpoint 3 detector, a detector as secondcheckpoint 4 detector and a detector as second trigger 5.

All detectors are placed at appropriate distances between them, the sumof the relative distance being sufficient to properly carry out theprocess. The distances may be adjusted depending on the size of thesubstrate being controlled. All detectors are connected to a circuit 14which is able and programmed to collect the information about therespective detection made by the detectors, use this information todecide whether or not the control of a given substrate has given theproper sequence of detection, and generate an error message ofnecessary.

The detectors are preferably optical detectors made of LED or otherequivalent light emitter, which are known in the art. They each furthercomprise a detecting element which is able to detect a change in thereflection of the light emitted by the diodes, due to a modification ofthe background, for example when the background changes from a darkbackground to a clear background (detection of the leading edge of asubstrate) or from a clear background to a dark background (detection ofthe trailing edge of a substrate), that is absence of substrate such apaper and presence of substrate.

A circuit 14 which can be used in the method according to the inventionis represented in FIG. 5 of the application. Derived from theabove-mentioned boolean operations, the circuit given as a logic circuitfulfills the needed function. Since the detection of a leading edge isthe Boolean negation of the trailing edge the output of the RS-flip-flopQ1,Q2 will give the result for the integrity of the trailing edgewhereas {overscore (Q1)},{overscore (Q2)} will give the result for theleading edge.

A simple trigger T can be used to reset the device when necessary.

A machine, for example a printing machine in the field of securities,may comprise at least one control device according the invention. Such acontrol device may also be placed at several positions of the machine inorder to control the integrity of the substrate, for example a sheet ofpaper, at different stages of the printing process.

The embodiments of the invention are given by way of example and are notto be considered as limiting on the scope of the claims.

1. Process for controlling the integrity of planar substrates, such assheets of paper, characterised by the following steps: providing a firsttrigger at a selected first location along the direction of displacementof the substrate for detecting the passage of an edge of the substrateat said first location; providing a second trigger at a selected secondlocation after said first trigger along the direction of displacement ofthe substrate for detecting the passage of an edge of the substrate atsaid second location; providing at least a first checkpoint detector ata selected third location between said first and second triggers alongthe direction of displacement of the substrate, said at least firstcheckpoint detector being adapted to detect the passage of said edge ofthe substrate at a selected place along said edge which is differentthan the place at which said first and second triggers are adapted todetect the passage of said edge; detecting the passage of said edge ofthe substrate at said selected locations by means of said first trigger,said at least first checkpoint detector and said second trigger;controlling whether the passage of said edge of the substrate wasdetected by said at least first checkpoint detector after detection bysaid first trigger and before detection by said second trigger; andgenerating an integrity check failed message in case the passage of saidedge of the substrate was not detected by said at least first checkpointdetector after detection by said first trigger and before detection bysaid second trigger.
 2. A process as claimed in claim 1, wherein two ormore checkpoint detectors are provided at selected locations betweensaid first and second triggers along the direction of displacement ofthe substrate, each of said checkpoint detectors being adapted to detectthe passage of said edge of the substrate at selected places along saidedge which are different that the place at which said first and secondtriggers are adapted to detect the passage of said edge.
 3. A process asclaimed in claim 2, wherein the integrity check failed message isgenerated in case the passage of said edge of the substrate was notdetected by one of said checkpoint detectors after detection by saidfirst trigger and before detection by said second trigger.
 4. A processas claimed in claim 2, wherein two checkpoint detectors are locatedclose to the corners of said substrate.
 5. A process as claimed in claim1, wherein said edge is the leading edge and/or the trailing edge of thesubstrate.
 6. A process as claimed in claim 1, wherein said detection ismade by optical means.
 7. A control device for controlling the integrityof planar substrates, such as sheets of papers, said device beingcharacterised in that it comprises: a first trigger arranged at aselected first location along the direction of displacement of thesubstrate for detecting the passage of an edge of the substrate at saidfirst location; a second trigger arranged at a selected second locationafter said first trigger along the direction of displacement of thesubstrate for detecting the passage of an edge of the substrate at saidsecond location; at least a first checkpoint detector arranged at aselected third location between said first and second triggers along thedirection of displacement of the substrate, said at least firstcheckpoint detector being adapted to detect the passage of said edge ofthe substrate at a selected place along said edge which is differentthan the place at which said first and second triggers are adapted todetect the passage of said edge; and a computer element adapted tocontrol whether the passage of said edge of the substrate was detectedby said at least first checkpoint detector after detection by said firsttrigger and before detection by said second trigger.
 8. A control deviceas claimed in claim 7, characterised in that it further comprisesanother checkpoint detector arranged at a selected fourth locationbetween said first and second triggers along the direction ofdisplacement of a substrate and which is adapted to detect the passageof said edge at another place along said edge.
 9. A control device asclaimed in claim 7, characterised in that said triggers and checkpointdetectors are optical detectors.
 10. A control device as claimed inclaim 7, characterised in that said triggers and checkpoint detectorscomprise light emitting diodes.
 11. A machine characterised by at leastone control device according to claim 7.